Composite archery bow



Jan. 12, 1954 F. B. BEAR COMPOSITE ARCHERY BOW Filed April 2l, 1950@galera/6 ewz" @www |Nc Hfs jy j .fiable mier Allllllllllllllllllllil.

Patented Jan. 12, `1954 2,665,678 ooMPosITE ARCHERY Bow Frederick B.Bear, Grayling, Mich., assignor to Bear Archery Company, Grayling,Mich., a corporation of Michigan Application April 21, 1950, Serial N o.157,213

Claims. (Cl. 124-23) This invention relates to archery and moreparticularly to bows, and has for an object the provision ofimprovements in bows and in the art of bow construction.

`In order to be eiective for the propulsion of an arrow, the activelimbs of a bow must be of resilient material. When a bow limb is bentlongitudinally, its concave side is subject to longitudinal compressionand its convex side is subject to longitudinal tension, according to thewell-known mechanical laws of beams.

Reduced to simplest terms, what is required of a bow is that it shallshoot an arrow. There are good, indifferent and bad bows; and all ofthem can shoot an arrow. In order to difierentiate among them it isnecessary to establish criteria of quality. Among these is a force-drawcharacteristic so unvarying that the total energy for propelling a shaftmay be the same in each successive shot and that the limbs of the bowmay work without sensible variation in their mode of motion insuccessive shots. There are also the highly desirable qualities oflightness in the hand and freedom from jar o1' kick at the instant thepropelled arrow leaves the string. All other factors being apparentlyequal, bows may be differentiated from one another by a criterion ofeffectiveness. One element of this criterion is called castf Cast isdened as that property of a bow which enables it to impart velocitytoan'arrow. It may be expressed in terms of that velocity which .a bowimparts to an arrow of stated mass and material. However, since arrowsof different masses, materials and stiinessor spine-acquire differentvelocities when shot from the same bow at the same drawlength, it iscustomarywhen comparing different bows as to cast-to choose aluminumarrows, of optimum spine in relation to the bow to be tested, becausethey fly Vmore consistently although slightly more slowly than dootherwise comparable wooden arrows; and to maintain a constant ratiobetween the mass of the test arrowv and the draw weight of the bow atthe most usual full'draw-length, which is 28 inches. For the presentpurposes, the ratioof 8 grains of arrow weight, at optimum spine, to onepound of draw-weight at full draw will beused.

Draw-length means the distance of the drawn string from the back of thebow, and not the lesser actual distance that the string is drawn fromits position-of-rest, or the so-called braced position.

`The term full draw as herein used shall be taken to mean that 'lengthofstring-'draw which will bring the pile, or head, of a hooked 28'incharrow to the back of the bow. AAll numerical values relating to bowaction -herein reported are referred to full draw at a draw-length ofv28 inches.

In all bows, cast is influenced by the stmela or distance from the faceof the handle of the bow to the string at the braced position. Theoptimum nstmele varies according to the geometry and construction of theparticular bow in question. Whenever cast, in terms of velocity, ishereafter mentioned, it is to be understood that the fistmele is at theoptimum.

In order that the mass and spine of the test arrow be the only sensiblevariables, aluminum arrows are used, as stated; and the dimensions ofthese arrows-including the number and character of the feathers and theheight, length and total area of the fletching-are kept constant.

For the present purposes, the reported. values of arrow velocity arethose representing the average velocity over a distance of 20 feet fromthe tip of the arrow-at the instant it leaves the string-- to the faceof the target. The precision of measure, by the method employed, isabout plus or minus 0.25%, Thus, in the range to be reported, adifference of one foot per second is a signincant gure.

In order to achieve accuracy in the shooting of successive arrows, theiraverage velocity over the distance from bow to target must be keptconstant to better than one foot per second. Hence, a variation inaverage velocity of one foot per secondY over the short test distanceis. of real significance to the archer. A change from normal to hot,humid weather may reduce the cast of a bow constructed of organicmaterial. Ane other factor that reduces cast is elastic hysteresis If abow is kept at full draw for an appreciable length of time-as tenseconds, for instance, the force `corresponding to the thus-producedamount of distortion ordinarily diminishes. The bow lets down, or losesdrawweight. This letdown is characteristic of all prior bows con?structed of organic materials, as wood, or laminations of wood and horn,sinew, silk or plastics.l If the 'let-down results in a permanent set ofthe limbs, thel bow is said to have followed the string. To the extentthat there is string-follow, there is anpermanent loss of cast. If,however, the let-down is of a transient nature and is one from which thebow substantially recovers within a short period of timeas between.successive shots, for instanceelastic hysteresis is involved.

Elastic hysteresis is a detrimental variable factor in the performanceof prior bows constructed of organic materials: variable because itsmagnitude is influenced by temperature and relative humidity; anddetrimental because of its variable reduction of cast.

Bows of all-metal construction do not exhibit sensible hysteresis. Steelbows date back at least several centuries and possibly more, inasmuch asthey are mentioned in the Bible. Specimens that fr have come down to usfrom the Persians, as well as their modern counterparts-whether tubularsteel or solid aluminum alloys--are not only heavy in the hand butnotoriously hard onthe bow-hand because they all exhibit to some degreea distressing kick or jar of recoil. Modern aluminum bows are usuallyprovided with a heavy metal handle that absorbs some of the jar yetnecessarily adds to the weight in the hand. The kick or recoil of a bowhas been found generally to be small in bows of good Vcast havinglight-weight limbs, and large n'sluggish bows having heavy limbs,

The production of a bow having maximum cast with a minimum weight ofmaterials-which is an important object of this inventioninvolves acompromise between using as little material as possible and yet as muchas is needed in order to avoid the hazards of breakage, and moreparticularly such breakage as may put the archer in jeopardy.

Bows may be classied as to type by the length between nocks whenunstrung and by the unstrung position of the limbs. Bows whose limbs, inthe unstrung position, are either straight and set backwards from thehandle or are curved backwards from the handle throughout any part oftheir active limb-length are called reflexed or pre-stressed.

The famous Turkish flight bow of several centuries ago is an example ofextreme pre-stressing by reflexing. These bows, especially whenshortlimbed, were very fast; that is, they had good cast. They andtheirV modern counterparts were designed primarily for maximum cast.Short, heavily pre-stressed bows are not suitable for precision shootingbecause 'of at least two reasons. First, all heavily pre-stressed bowsare extremely sensitive to minor variations in the archers loose orrelease of the string from his drawing fingers, and hence they tend tomagnify these variations with consequent loss of consistency inperformance. Second, when the limbs and connecting handle are short, theangle made by the string at the archers ngers at full draw is very acuteand tends to increase the difliculty of his making successivelyconsistent and clean releases.

The standard length of the famous English longbow was approximately '72inches. The Turkish flight bow was usually about Lifi-48 inches inlength along the bow from nock to nock, as are the present day flightbows. The modern target and hunting bows, to improvements in which thisinvention relates, are in the range of 58-68 inches. Hunting bows areusuallly towards the lower part of this range, while the precisiontarget bows are usually in the upper part.

In a beam of homogeneous material and of substantially rectangular ortransversely symmetrical cross-section, the neutral layer is midwaybetween the tension and compression sides. This condition holds for theall-metal bows in which the amount of stretch of the surface material onthe tension side or back of the bow is 4 substantially equal to theamount of compression of the face, or belly side, of the bow.

In bow woods, such as yew and osage orange for instance, the strength intension is greater than that in compression. Bows made of these woodstend to fail in compression by a crushing of the bers on the belly side;this is called chrysallingJ There hasl been a marked tendency in recentyears to still further increase the strength of the back of the bow bythe application thereto of materials having greater tensile strengththan wood. For many centuries sinew has been used as a backing, notablyin the Turkish bow where the high tensile value of the sinew wasbalanced by the high compression value of a horn facing. More recently,silk threads and glass fabrics have been adhesively fastened to thebacks of bows as tension elements. Such backings greatly increase thecompressive strain on the belly of the bow and therefore increase thehazard of failure through chrysalling. Even sheet steel and steel wirehave been suggested as a backing for wood bows, notwithstanding the factthat there is no known bow wood which could possibly balance incompression the high tensile values of steel.

The effect of greater strength in tension than in compression-due to theapplication of backing materials highly resistant to stretch-is that thesurface of the belly of the bow is compressed more than the back of thebow is extended under tension. The crushing effect of excessivecompression not only shortens the life of the bow by chrysalling butendangers it in still another manner. While, as stated, the tensionvalue of bow woods is generally greater than their compression value,any crushing of the surface fibers of the belly side greatly lowers thetension value of that side as well. The tension value of the fibers ofthe belly side of the bow is of importance in ordinary use because, whenthat side is longitudinally compressed, it is also subjected tocorrespondingly heavy transverse tension. This is the so-called St.Venon eifect. Failure in transverse tension on the belly side results inlongitudinal hairline cracks.

When, as sometimes accidentally happens, the string of a fully-drawn bowis loosed without an arrows being nocked thereon-as, for instance,because of a defective or broken arrow nookthe bow must dispose of allits stored-up energy unaided; and it almost invariably breaks throughtension failure on the belly side.

It is an object of this invention to provide a bow having a backing anda facing, said facing being characterized by greater resistance tolongitudinal compression than the resistance of said backing tolongitudinal tension; and further characterized by its ability towithstand, without breakage, the tension stresses imposed upon it by therelease of the string at full draw without the energy-absorbing presenceof an arrow engaged therewith.

For any bow the draw characteristics may be shown by a curve plottedwith force as the ordinates and the length of draw as abscissas. This isthe so-called force-draw curve, the shape of which depends mainly uponthe dimensions and the geometry of the bow. Many prior bows werecharacterized by an increase in the upward slope of the force-draw curvejust before full draw. This so-called stacking up at the last of thedraw is not only detrimental to accuracy but very unpleasant to thearcher. Since the energy input and therefore the available energyaeeae'ls are represented by the area under the force-draw curve, it isevident that Athe last increment of draw contributes vastly more to thestored energy than does `an increment of draw near the beginning. Fromthis it is apparent that the slightest diiference in the length of drawwill make a large difference in the resulting velocity of the arrowrelease of the string.

Accuracy is best obtained when the force-draw curve is linear in thatportion approaching and passing through full draw. Under the heavystrain on the drawing iingers of holding a bow full-drawn while thearcher is taking aim, there is a tendency for the fingers to creepforwards; this is highly detrimental to accuracy. This tendency is at aminimum when the force-draw curve is linear approaching and through fulldraw. Such a bow is said to have a smooth draw."

Assuming that a bows lightness in the hand, its sweetness or relativelysmall jar or recoil, its smoothness of draw and its over-all length inrelation to its draw-length are all at the optimum, there is still thequestion of its effectiveness in terms of cast.

The traditional English longbow had a cast corresponding to a velocityin the neighborhood of 165 feet per second. The modern steel bows impartto an 8:1 arrow velocities in the range of 1'75-180 feet per second. Themodern aluminumalloy bows correspondingly impart velocities in the rangeof 180-190 feet per second. The fastest modern all-wood bows, so far asknown, are comparable to those of the famous hunting archer, HowardHill, and the equally renowned bowyer, Russell Willcox. The rst of theseis a f straight bamboo bow 70 inches long; the second is an osage orangebow backed with a thin lamina of hickory, highly reflexed towards thetips, and 63 inches long. They both impart a velocity of 180 feet persecond to an 8:1 arrow. The Willcox bow is illustrated opposite page 241in Elmers Modern Archery.

The fastest modern prior composite bows, so far as known, are backedwith two plies of glass fabric over a subbacking of plastic and facedwith a layer of plastic, They range in length from 62-67 inches, andimpart velocities to an 8:1 arrow within the range of 190-192 feet persecond.

Considering that the improvements in the cast of bows-over the lasthundred years or morecorresponds to a velocity increment of less than itis obvious that to obtain a further significant increment of 2% or morecannot be viewed as a mere matter of degree, for such an improvementwould be a real advance in the art. Bows according to this inventionhave exhibited cast in the range of 198 to 205 feet per second.

It has long been held that a fully-drawn bow of good cast is nine-tenthsbroken; and no archer up to now would permit his bow to be drawn beyondthe length for which yit was designed. The breaking of a bow at fulldraw can have serious consequences to the archer involved. With theadvent of the modern all-metal bowsand more particularly thealuminum-alloy bows, all of which tend to fail through vibrationalfatiguethe consequences of breakage can be, and in fact have been,serious indeed. The always sharp and sometimes jagged edges of thebroken metal moving at high speed are far more dangerous than are thebroken ends of a wooden bow.

One of the offsetting advantages of the modern all-metal bows is thatthey are -completely unaffected by moisture and are practicallyunaffected by change in temperature over the normal range in which a bowmay be used; whereas the draw weight of Wooden bows-o1' of compositebows, in which either or both the tension and compression elements areof organic origin-may change as much as, or even more than, 1% for every10 degrees Fahrenheit change in ambient temperature, with acorresponding though not directly proportional change in cast. Thesebows notoriously let down in hot, humid weather and tend to more or lessfollow the string, or lose draw-weight from much shooting. They alsohave the detrimental characteristic of temporarily gaining draw-weightat low ambient ternperatures. Thus, a bow having a draw-weight suitableto its user at normal temperatures may so increase in draw-weight attemperatures encountered in hunting (as 0 F., for instance) that theuser may no longer be able to bring it to full draw. And, even if he didsucceed in so doing, he would not be able from experience to accuratelyjudge its cast and hence the trajectory of the arrow.

It is an object of this invention to provide a composite bow in thelength range of 58-68 inches and therefore suitable for hunting andprecision target shooting, which bow is characterized by a substantiallygreater elongation of its tension element than shortening of itscompression element at full draw; and further characterized by the factthat it can be drawn several inches beyond the normal draw-length of 28inches without breakage; and still further characterized by the factthat, when fracture does occur-as after 25,000-30,000 shots, forinstance-it does not involve rupture of the compression element andtherefore completely eliminates the danger Vto the archer heretoforealways associated with bow breakage at full draw; and furthercharacterized by its ability-at a draw length of 28 inches-to impart toan arrow of optimum spine and having a mass of 8 grains for each poundof bow-draw-weight at 28 inches an average velocity over a distance of20 feet of at least 195 feet per second.

It is another object of this invention to provide a composite bow havinga force-draw curve which is linear from not more than 25 inches through28 inches to at least 29 inches; and further characterized by asubstantially imperceptible hysteresis or loss of cast when held at fulldraw for one minute at normal ambient temperatures; and still furthercharacterized by such immeasurably small string follow as is reflectedin the loss of not more than 0.5% of its original drawweight for every5000 shots.

It is another object of this invention to provide a composite bow,comparable in mass to an all-wood bow of like draw-weight and length,characterized in that both the tension and the compression elements areof resilient material having a modulus of elasticity greater than wood.

Other objects of the invention will appear as the description thereofproceeds.

Glass-fabric, as heretofore used as a tension element, has customarilybeen laminated to a thin subbacking layer of a thermosetting plasticresin, hereinafter referred to as a plastic. It has been stated, by theproponents of the plastic subbacking for glass-fabric bow backings, thatthe plastic was necessary because the difference in tensile-shearcharacteristics between the glass-fabric laminate andthe wood core ofthe bow was so great that, without it, the wood was prone to `fail at ornear the glue jointor the core Vand backing interface.

One of the features of this invention is based uponr the discovery thatwhen the compression element is made to consist of a thin layer of asuitable metal, sensible hysteresis is eliminated and a substantiallybetter cast is obtained without rather than with theheretofore-considered essential plastic subbacking for a multi-plyglassfabric backing.

However, the advantageous elimination of the prior plastic subbackingintroduced a real diiliculty which relates to the fact that thethermosetting cements most suited to the bonding of glass bers andfabrics to one another require a heat and pressure treatment that isdetrimental to the preferred core woods. Moreover, suitable oements forglass-fabric tension elements are not adhesively wet, and hence cannotbe satisfactorily bonded, by adhesives-which set at such-lowtemperatures as are required to preserve the maximum strength of thecore wood.

-Obviously, a subbacking is required; and, for reasons which willappear, it should have the same coeicient of expansion as the glass orshould be so yielding as not to sensibly pre-stress the glass when laterapplied to the bow limb as a backing.

Surprising as it may seem, it was discovered as part of this inventionthat a thin lamina of wood, and preferably maple, was found to answerthe requirements when it was so bonded to the plies of glass-fabric thatthe cement penetrated the wood lamina to an appreciable fractionalthough less than half its thickness. When this backing with itspartially permeated subbacking or" wood was bonded to the core wood by asuitably lowtemperature setting cement, the latter could penetrate to,or nearly to, the line of penetration of the first-appliedhigh-temperature cement. Both cements thus achieve a good bond, and tendalso to reinforce the thin layer of wood against tensile-shear failurenotwithstanding the deleterious effect upon the wood subbacking of thehigh temperature treatment to which it has been subjected.

As stated, pre-stressed bows tend to have improved cast, but at theexpense of accuracy of performance. Heretofore, all of the compositebows incorporating glass-fabric as the major tension element have beenmore or less heavily prestressed. Their exceptionally good cast, ashereinabove noted, is in part due to the pre-stressing obtained byeither setting straight limbs back from the handle or by reflexing,although sometimes by a combination of both-in which case the set-backlimbs are generally straight for the major portion of their length andheavily reflexed toward their ends. These refiexed ends are commonlycalled working recul-ves in order to distinguish them from the stiffunbending ears of the common recurve. In addition to the soobtainedpre-stressing, still more is attained through stretching theglass-fabric over the straight portion of the limb.

In a multi-ply backing, comprising laminations of glass-fabric, it isobvious that when flexed the outer layer on the convex side is normallyput under greater tension than is the next inner layer, and so on-ifthere be more than two layers of glass-fabric. The two layers of theprior glass backing do not carry an equal share of the tension load.

It is a further object of this invention to provide prelaminatedbackings comprising a plurality of glass-fabric layers greater than two,which plurality when incorporated in a bow is so arranged that-at therst part of the draw-the tension load on the several layers of glassfabric is greatest on the innermost layer, and increases relatively onsuccessive layers as the draw progresses until the tension load on alllayers is substantially equal at approximately three-quarters of fulldraw: which is of material advantage.

The wood core of a composite bow is put under tremendous strain by theforces acting upon it. The great shearing stresses increase toward thetip, as is well understood. The complicated vectors of tension andcompression stresses acting upon the core are not so clearly understood.When, as in the case of the bows of this invention, the modulus ofelasticity-' of both the tension and compression elements-is very highrelative to Wood, the' resulting stresses on the core wood arecorrespondingly very great.

Among some 4000 bows made according to the method of this invention, andsold, there werea few which developed a more or less pronounced dogsleg, or hinge on one limb, with a 'consequent substantial let-down (lossin drawweight) although no corresponding or even substantial loss incast was, when the bow was measured at the resultant draw-weight, foundto have resulted. Careful inspection failed to disclose any localweakness in either the tension or the compression elements.

It was thought that there might be an invisible crushing of the corewoodbers. Whether or not this was so is not known. vIn an endeavor, however,to increase the shear strength of the core, a discovery of a surprisingnature was made which provided a solution of the problem presented bythe occasional occurrence of hinges.

In the endeavor to increase the shear strength of the core beyond thatof otherwise suitable woods, cores were made of thin laminations of wood(about 5/64 thick) with the grain of alternate layers at right angles toone another, the assembly being then cut atan angle of 45 to the grain,into strips suitable for the core. Using these bias-cut plywoods, grainon edge (that is, making an angle of 45 with the back and belly of thebow), it was found that the shear strength was some 50% greater thanthat of a comparable normal strip of the same wood having the shearstresses parallel to the grain. The surprising and wholly unexpectedfinding was that the prior occasional, inexplicable and detrimentalhinges did not occur when the bias-cut core was used on-edge. Manyspot-check tests-to-destructicn failed to disclose any indication ofhinges in the limbs of bows so tested.

The core of composite bows, having tension and compression elements ofgreater modulus of elasticity than wood, serves primarily to coherentlyhold these elements spaced apart at precisely predetermined distances.equal, the distance these elements are spaced apart, at any point',determines the stiffness of the bow limb at that point.

The width of the bow limb and its taper toward its outer tip is also animportant factor in its stiffness.

When the tension and compression elements must each be held to constantthickness throughout their length by reasons of practicality, economy orother production considerations, the stiliness of the limbor draw-weightof the bowmust depend upon the variables or" the width of the limb, thethickness of the core and the taper` Everything else being aeoaees ofthe formerand preferably though not always nor necessarily of the latteras well.

Moreover, adjustments in the stiffness of the limbs to bring aboutsubstantially uniform stress distribution along the length of thelimband to bring about a balanced action of both limbscalled tilleringthe bow-must be made on the edges of the limbs only.

As vbetween. two composite bows having the same width and shape of limbsand the same strength in their tension elements and in their compressionelements, the draw-weight of the bow having the thinner core will beless than that of the otherwise like bow having a thicker core.

In the 50-ponnd draw-weight range, a change in core thickness ofthousandths of an inch near the base of the active portion of the vlimbmay change the draw-weight by as much as 5 pounds in a bow madeaccording to this invention, l

As is well recognized in the art of bowmaking, the handle section of abow should be unbending. The block of wood customarily applied to thebelly at the handle section is called the risen The riser is usuallytapered at both ends, more or less abruptly, down to the limbs.

If the riser has a very short taper, the resulting increase in stiffnessalong the taper tends to be very abrupt. The modern metal bows havehandle-sections into which the limbs are in serted, which sections endabruptly so that the active portion of the bow limb terminates inwardlyat a sharply dened line corresponding to the outward end of the handle.Hence, the limbs of all-metal bows arestiiened abruptly by theirinsertion into heavy metal handles. This results in nodal vibration andlocalized vibrational fatigue. i

As stated, the limbs of metal bows and particularly those ofaluminum-alloy bows fail through vibrational fatigue.

It is an object of this invention to provide a bow having as acompression element a facing of thin metal, which bow is characterizedby the i substantial absence of vibrational fatigue of the metalcompression element. l

In prior wood and composite bows, the risers have sometimes been taperedto as much as three inches, and in at least one prior bow they have beentapered to four inches.

The taper of the risers of the bows of this invention are preferablyasymptotically tapered about 100% longer than the longest taper in anyknown prior bow. The purpose of the very long asymptotic taper of thebows of this invention is much more gradually to increase the stiifnessof the base of the bow limb than has heretofore been either customary orrequired in bows comprising compression elements of organic material,and thereby to avoid nodal vibration in the bow limb and thus toeliminate the localized detrimental effects of vibrational fatiguecharacteristic of all-metal bow limbs.

Conventionally, the riser is applied to the belly side of the bow limbover the tension element.`

The zone of least stress in a bow limb is at the base of the limb in theso-called neutral layer transverse of the limb. vIt has been found byexperience, as part of this invention, that best results are obtainedwhen the riser is carried down into the core at the approximate locationof the neutral layer. This is most conveniently 4accomplished by makingthe core of two laminates of equal thickness, by separating the two atthe base of the limb by the dip of the riser and by carrying the corelamina on the belly side with its adherent compression facing up overthe taper of the riser.

Instead of snubbing the vibration of the bow limbs suddenly at theirjunction with the bow handle, as in prior metal bows, the action of thebow limb of this invention is 'gradually damped by the bending of thelong flexible extension of the dips. The result is that, in all the manylife tests-to-destruction that have been made on bows of this invention,there has never been an instance in which evidence of vibrationalfatigue of the metal compression element has been observed.

As has been stated, the longitudinal elongation of the outer surface ofthe tension element of prior bows has been either equal to thelongitudinal shortening of the outer surface of the compressionelement-as in the case of all-metal bows-or the elongation of the formerhas been substantially less than the shortening of the latter, as inwooden bows and in composite bows having organic compression elements.

`So far as is known, no prior composite bow has ever been made in whichthe longitudinal deformations of the backing and facing due to tensionand to compression have been equal. The longitudinal deformation ofcompression has always been greater than the deformation due tolongitudinal tension.

The lea-st difference found in prior composite bows corresponds-to 7.7%more shortening than extension. The maximum difference found in priorcomposite bows having a glass-fabric tension element and a woodcompression element corresponds to 58.3% more shortening than extension.For a wide variety of prior bows of allwood or of composite constructioninvolving an inorganic tension element and organic compressionelement-as a plastic, for instance, the average shortening undercompression (at full draw in each case) was found to correspond to 35.4%more shortening than extension.

As stated, it is an object of the present invention to produce acomposite bow, having essentially inorganic tension and compressionelements, characterized when stressed by an 'elongation of the tensionelement at least equal to or greater than theshortenng of thecompression element. l

From the standpoint of optimum distribution of the shear stresses withinthe core of the bow,

the elongation` and the shortening should be substantially equal. Fromthe standpoint of the safety of the archer, in case the bow should breakat lfull draw,and for the unforeseen and surprising reason oftheobtained substantial improvement in cast, it `is desirable that thecompression element should shorten at full draw less than thetensionelement elongates and that the compression element should yieldbut not break when and if the tension element fails.

In order to achieve an outstanding improvement in cast and at the sametime to obtain the desired margin of safety for the archer, it has beenfound advantageous-as part of this invention--to compromise in favor ofsafety to the archer as against optimum distribution of internalstresses.

This advantageous compromise has been effected through the novelcombination in a bow of a tension element comprising a thin layer ofresilient material having a modulus of elasticity greater than wood,spaced apart by a wooden core from a thin*sheet-metalcompressionelementvsuch as spring steel, beryllium bronze or a hard aluminum alloy.

The eect of so doing has been Yto produce for the rst time bows in whichthe elongation of the tension element at full draw is greater than theshortening of the compression element up to the order of 35% butpreferably in the neighborhood of to 10%. Y

Aand an intermediate core element, the elements being interconnectedthroughout their interfaces with adhesive, and the outer surfaces of thetension and compression element being approximately parallel at anycross-section throughout substantially the active length of the limb,characterized in that the tension element comprises a thin' layer ofresilient material having a modulus of elasticity higher than wood,preferably glass threads extending lengthwise of the limb, either withor without cross threads interwoven therewith, the core member compriseswood, preferably maple, and the compression element comprises a thinribbon of resilient metal having an elastic yield factor whichapproaches but does not substantially exceed that of the tensionelement, the elastic yield factor being the change in the length of theelement in drawing the bow, the tension element temporarily increasingin length and the compression element temporarily decreasing in length.Intermediate the metal compression element and the wood core is asubfacing which preferably comprises a thin strip of maple or the likewhich is impregnated on its opposite sides with two differentthermoplastic adhesives respectively, a relatively high temperatureadhesive for adhesion to the metal and a relatively low temperatureadhesive for adhesion to the wood core. The sub-facing is attached tothemetal compression element rst at a temperature higher than that whichthe wood core could safely withstand, and then the combined compressionelement and sub-facing are joined to the wood core at a lowertemperature which the wood core can safely withstand.

In a more specific aspect the bow comprises a riser having dips whichmeet the back yof the riser substantially asymptotically at the ends ofthe riser, the compression element and subfacing extending along theface of each limb of the bow to the riser and thence along the dip tothe inner end of the dip, terminating substantially at the gripintermediate the two dips. A portion of the core element may also extendalong the dip to the junction between the grip and dip. Preferably thecore of each limp comprises two layers which extend in juxtapositionfrom the free end of the limb to the end of the riser and thence alongthe face and back side of the riser, the face layer terminatingsubstantially at the inner end of the dip. This construction eliminatesnodal virbation and therefore minimizes fatigue in the metal of thecompression element.

In still another aspect of the invention the wood core compriseslaminations extending in planes parallel to the plane defined by the'longitudinal axis of the bow, the grain of each of the laminationsextending obliquely with respect to the face of the bow and the grain inalternate laminae extending transversely of each other. In the preferredembodiment the core comprises two layers and the grain of eachlamination of each layer extend at an angle of approximately 45 withrespect to the face of the bow and the grain in alternate laminae ofeach layerextend at approximately right angles t0 each other. vThisconstruction avoids the danger of the aforesaid dogs legs by eliminatinglocal weak spots sometimes occurring in normal core woods andsubstantially increases the resistance of the core to the greatlyincreased tension-shear stresses put upon it by the high modulus ofelasticity of both the compression and tensionrelements.

In a further aspect of the invention the tension element comprises aplurality of strata, the outer stratum being longitudinally compressedwhen the element is attached to the core so that as the bow is exed inbracing the compression is reduced. Preferably the different stratacomprise different layers adhesively joined together. In the preferredembodiment the outer stratum is not only longitudinally compressed butthe inner stratum is longitudinally tensioned when the element isattached to the core. rIhe preferred degree of compression and tensionin the outer and inner strata is such that at threequarters of vfulldraw the innerand outer strata are subjected to approximately equaltension.

For the purpose of illustration a typical embodiment of the invention isshown in the accompanying drawings in which Fig. 1 is a side elevationof a braced bow of the type having unbending ears at the free ends ofthe limbs;

Fig. 2 isa rear elevation with the upper end broken off;

Figs. 3 and 4 are longitudinal sections on lines V3 3 and Q- of Fig. 2;

Figs. 5, 6 and 7 are cross-sections on lines 5 5, 6-6 and l-1 of Fig. 1

Fig. 8 is a longitudinal section through the end of a multi-layertension element at the outer end of the element before it is applied tothe core;

Fig. 9 is asimilar section of a tension element for use in making aconventional type of bow which does not have unbending ears; and

Fig. lo comprises force-draw curves.

'I'he particular embodiment of the invention chosen for the purpose ofillustration comprises a string l, a riser 2 having dips 3, a grip 4surrounding the central portion of the riser, atension element 6 on theback side of the bow, a compression element 'I on the face or belly sideof each limb of the bow, a core comprising two layers 8a and 8bintermediate the tension and compression elements, a sub-backing ilbetween the tension elementi; and the core, a sub-facing l 2 between thecompression element and core and a nish layer I3 of wood or otherdecorative material over each compression element. The various layersare secured together with thermosetting adhesives appropriate to thematerials at the different interfaces, the sub-backing H and thesub-facing l 2 being joined to the tension and compression elementsrespectively at relatively high temperature before these two elementsare joined to the wooden core at relatively low temperature. Thecompression elements may be formed of any one of several metals. When ffa formed of spring steel they are preferably approximately 0.018 to0.022 inch thick, when formed of beryllium bronze they are about 0.029to 0.037 inch thick, and when formed of hard aluminum they arepreferably about 0.051 to 0.064 inch thick. The sub-facing andsub-backing are preferably formed of thin strips of maple impregnated onopposite sides with high-temperature and low-temperature adhesivesrespectively. For the high-temperature adhesive best results have beenobtained with phenolic resin or a combination of phenolic and polyvinylresins. After the strips of maple have been impregnated part way throughfrom one side and then dried, they are applied to the tension andcompression elements at a temperature of about 325 F. with a pressure ofabout 150 pounds per square inch for about 30 minutes. Thereafter thesub-facing and sub-backing are impregnated part way through from theirexposed sides with a low-temperature urea formaldehyde cement and thenapplied to opposite sides of the core and riser at a temperature ofabout 180 F. un der a pressure of about 80 pounds per square inch forabout 60 minutes.

As shown in Fig. 3 each dip 3 of the riser 2 meets the back of the riserasymptotically at one end of the riser, and both the compression element1 and the sub-facing I2 extend along the face of the dip to the innerend of the dip adjacent the grip.` The tension element E and thesub-backing H preferably extend continuously across the back of theriser. In the preferred embodiment the two layers of the core divergefrom each other from the end of the riser to the grip, one layerextending along the back of the riser approximately to themiddle pointI4 of the riser and the other layer extending along the face of the dipto the inner end thereof.

As shown in Figs. 5 and 6 each layer of the core 8 comprises laminationswhose interfaces lie in planes parallel to the plane containing thelongitudinal axis ofthe bow. As shown by the marking in the longitudinalsection of Fig. 4, the grain of each lamination of each layer extends atapproximately 45 with respect to the face of the bow and the grain inalternate laminae of each layer extend in opposite directions so thatthe grain in alternate laminae of each layer extend approximately atright angles to each other.

AS shown in Figs. 4 to 8 inclusive the tension element 6 comprises aplurality of layers, more than two and preferably ve, cemented together.As shown in Fig. 8 the inner layers 6a` and 6b terminate short of theother layers 6e at` the free ends of the limbs, and the layers arecemented together and to the sub-backing While curved more than the coreso that the outer surface of the outer layer te is convex and the innersurface of the inner layer 6a is correspondingly concave. Then, when thetension element isfjoined to the corethrough the medium of thesub-backing `l l, it is flexedinto parallelism with the core, therebyputting the outer layers under compression and the inner layers undertension. The longitudinal curvature of the layers is preferably the sameas that of the bow limb at three-quartersfull draw so that atthree-quarters full draw both the inner and outer layers are subjectedt0 tension in `approximately the same degree. The tension elementcomprises layers of glass fabric having interwoven fibers extending bothlengthwise and crcsswise of the bow. However it matr comprise onlylongitudinal fibers cemented toffm gether in the aforesaid curvedformation so that, after the tension element is attached to the core,the outside strata are under compression and the inside strata are undertension.

As shown in Fig. 9 all of the layers of the tension element preferablyextend to the end of each limb when the aforesaid unbending ears are notused on the ends of the limbs.

In Fig. 10 the curve O illustrates the objectionable upward curvature inthe region of full draw characteristic of many types of prior laminatedbows and the curve N shows the desirable straightness n the region offull draw characteristic of the present bow.

A salient feature of the present invention consists in that the tensionand compression elements are so constituted and constructed that theelastic yield factor of the compression element approaches but does notsubstantially exceed that of the tension element. For most uniformdistribution of internal stresses the elastic yield factors should beapproximately equal so that at full draw the elongation of the tensionelement is about the same as the contraction of the compression element.However, to afford a higher degree of safety t0 the archer and yetobtain excellent cast, the elastic yield factor of the tension elementis preferably 5% to 10% greater than that of the compression element.

Bows according tc this invention exhibit substantially no hysteresis asdemonstrated by the fact that the cast at a draw dwell of five sec ondefor aiming is the same asat a draw dwell of sixty seconds Within theaforesaid precision of measurement.

Another advantage of the present bows is that they are substantiallyunaffected by changes in ambient temperature and relative humidity overthe normal range encountered in ordinary use.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within` the scope of the appended claims.

I claim I. i

1. An archery bow comprising a riser having dips which meet the back ofthe riser substantially asymptotically at the ends of the riser, limbswhich are wider than thick, each limb comprising -a tension element onthe back side, a compression element on the face side, an intermediatecore element intermediate the compres sion and core elements, saidcompression element terminating substantially at the inner end of thedip, said elements being interconnected throughout their interfaces withadhesive, the outer sul faces of the tension and compression elementsbeing approximately parallel at any crosssection throughoutsubstantially the4 active length of the limb, the tension elementcomprising a thin layer of resilient material having a modulus ofelasticity substantially higher than wood, the core member comprisingwood, and the compression element comprising a thin ribbon of resilientmaterial having an elastic yield factor which approximates that of thetension element.

2. An archery bow comprising a riser having dips which meet the baci. ofthe riser sub-stan.- tially asymptotically at the ends of the riser,

limbs which are wider than thick, each limb comprising a tension elementon the back side, a compression element on the face side, anintermediate core element and a sub-facing intermediate the compressionand core elements, said compression element and sub-facing terminatingsubstantially at'the inner end of the dip, said elements beinginterconnected throughout their interfaces with adhesive, the outersurfaces of the tension and compression elements being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisingAa thin ribbon of resilient material having an elastic yield factorwhich approximates that of the tension element.

3. An archery bow comprising a riser having dips which meet the back ofthe riser substantially asymptotically at the ends of the riser, limbswhich are wider than thick, each limb comprising a tension element onthe back side, a compression element on the face side and anintermediate core element, each of said cores comprising two layersextending in cemented juxtaposition from the free end of the limb to theend of the riser and thence along the face and back side of the riser,said compression element and face layer terminating substantially at theinner end of the dip, said elements being interconnected 4throughouttheir interfaces with adhesive, the outer faces of the tension andcompression elements being approximately parallel at any cross-sectionthroughout substantially the active length .of the limb, the tensionelement comprising a thin layer of resilient material having a modulusof elasticity substantially higher than wood, the core member comprisingwood, and the compression element comprising a thin ribbon of resilientmaterial having an elastic yield factor which approximates that oi thetension element.

4. An archery bow compris-ing a riser having a grip and dips which meetthe back of the riser substantially asymptotically at the ends of theriser, limbs` which are wider than thick, each limb comprising a tensionelement on the back side, a compression element on the face side, anintermediate core element and a sub-facing intermediate the compressionand core elements, each of said cores comprising face and back layersextending in cemented juxtaposition from the free end of the limb to theend of the riser and thence along the face and backside of the riser,said compression element and sub-facing and face layer terminatingsubstantially at the inner end of the dip, said elements beinginterconnected throughout their interfaces with adhesive, the outerfaces of the tension and compression elements being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisinga thin ribbon of resilient material having an elastic yield factor whichapproximates that of the tension element.

5. An archery bow comprising limbs which are wider than thick, each limbcomprising a tension element on `the back side, a compression element onthe face side and an intermediate core element, the elements beinginterconnected throughout their interfaces with adhesive, the outersurfaces of the tension and compression element being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisinga thin ribbon of resilient material having an elastic yield factor whichapproximates but does not substantially exceed that of the tensionelement, said core comprising two layers cemented together, each layercomprising laminations extending in planes parallel to the plane definedby the longitudinal axis of the bow, the grain of each of saidlaminations extending obliqiuely with respect to the face of the bow,and the grain in alternate laminae of each layer extending transverselyof each other.

6. An archery bow comprising limbs which are wider than thick, each limbcomprising a tension element on the back side, a compression element onthe face side and an intermediate core element, the elements beinginterconnected throughout their interfaces with adhesive, the outersurfaces of the tension and compression element being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisinga thin ribbon of resilient material having an elastic yield factor whichapproximates but does not substantially exceed that of the tensionelement, said core comprising two layers cemented together, each layercomprising laminations extending in planes parallel to the plane definedby the longitudinal axis of the bow, the grain of each of saidlaminations extending at an angle of approximately 45 with respect tothe face of the bow, and the grain in alternate laminae of each layerextending at approximately right angles to each other.

7. An archery bow comprising limbs which are wider than thick, each limbcomprising a tension element on the back side, a compression element onthe face side and an intermediate core element, the elements beinginterconnected throughout their interfaces with adhesive, the outersurfaces of the tension and compression elements being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisinga thin ribbon of resilient material having an elastic yield factor whichapproximates but does not substantially exceed that of the tensionelement, said tension element comprising a plurality of strata, theouter stratum being longitudinally compressed throughout itsthickness sothat as the bow is ilexed in bracing the compression is reduced.

8. An archery bow comprising limbs which 'are wider than thick, eachlimb comprising a tension element on the back side, a compressionelement on the face side and an intermediate core element, the elementsbeing interconnected throughout their interfaces with adhesive, theouter surfaces of the tension and compression elements beingapproximately parallel at any cross-section throughout substantially theactive length ofthe limb, the tension element comprising a thin layer ofresilient material having a modulus of elasticity substantially higherthan wood, the core member comprising wood, and the compression elementcomprising a thin ribbon of resilient material having an elastic yieldfactor which approximates but does not substantially exceed that of thetension element, said tension element comprising a plurality of strata,the outer stratum being longitudinally compressed throughout itsthickness and the inner stratum being longitudinally tensionedthroughout its thickness so that as the bow is flexed in bracing thecompression and tension are reduced.

9. An archery bow comprising limbs which are wider than thick, each limbcomprising a tension element on the back side, a compression element onthe face side and an intermediate core element, the elements beinginterconnected throughout their interfaces with adhesive, the outersurfaces of the tension and compression elements being approximatelyparallel at any cross-section throughout substantially the active lengthof the limb, the tension element comprising a thin layer of resilientmaterial having a modulus of elasticity substantially higher than wood,the core member comprising wood, and the compression element comprisinga thin ribbon of resilient material having an elastic yield factor whichapproximates but does not substantially exceed that of the tensionelement, said tension element comprising a plurality of strata, theouter stratum being longitudinally compressed throughout its thicknessand the inner stratum being longitudinally tensioned throughout itsthickness to such extent that at three-quarters of full draw the innerand outer strata are subjected to approximately equal tension.

10. In making a bow having a curved core, and a multi-layer tensionelement on the back side of the core, the method of manufacture whichcom-l prises adhering the layers of the tension ele--` ment togetherwhile they are curved more than the core, partially straightening thetension ele' ment so that its inner layer which is to be joined to thecore is under tension and its outer layer is under compression, and thenadhering the tension element to the core.

FREDERICK B. BEAR.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 1,605,300 Thompson Nov. 2, 1926 2,100,317 Hickman Nov. 30,1937 2,305,285 Ullrich Dec. 15, 1942 2,316,880 Miller Apr. 20, 19432,361,068 Sollid Oct. 24, 1944 2,415,881 Heftler Feb. 18, 1947 2,423,765Folberth et al July 8, 1947 2,428,325 Collins Sept. 30, 1947 2,479,342Gibbons et al Aug. 16, 1949 2,483,568 Waite Oct. 4, 1949 FOREIGN PATENTSNumber Country Date 627,255 Great Britain Aug. 4, 1949 OTHER REFERENCESGlass Reinforcements for Archery Bows." pages 5 and 6 of AmericanBowman-Review of January 1946, vol. 15, No. 6.

